Electrical resistivity and conductivity
Electrical resistivity and its converse, electrical conductivity, is a fundamental property of a material that quantifies how it resists or conducts the flow of electric current. A low resistivity indicates a material that allows the flow of electric current. Resistivity is represented by the Greek letter ρ; the SI unit of electrical resistivity is the ohm-metre. For example, if a 1 m × 1 m × 1 m solid cube of material has sheet contacts on two opposite faces, the resistance between these contacts is 1 Ω the resistivity of the material is 1 Ω⋅m. Electrical conductivity or specific conductance is the reciprocal of electrical resistivity, it represents a material's ability to conduct electric current. It is signified by the Greek letter σ, but κ and γ are sometimes used; the SI unit of electrical conductivity is siemens per metre. In an ideal case, cross-section and physical composition of the examined material are uniform across the sample, the electric field and current density are both parallel and constant everywhere.
Many resistors and conductors do in fact have a uniform cross section with a uniform flow of electric current, are made of a single material, so that this is a good model. When this is the case, the electrical resistivity ρ can be calculated by: ρ = R A ℓ, where R is the electrical resistance of a uniform specimen of the material ℓ is the length of the specimen A is the cross-sectional area of the specimenBoth resistance and resistivity describe how difficult it is to make electrical current flow through a material, but unlike resistance, resistivity is an intrinsic property; this means that all pure copper wires, irrespective of their shape and size, have the same resistivity, but a long, thin copper wire has a much larger resistance than a thick, short copper wire. Every material has its own characteristic resistivity. For example, rubber has a far larger resistivity than copper. In a hydraulic analogy, passing current through a high-resistivity material is like pushing water through a pipe full of sand—while passing current through a low-resistivity material is like pushing water through an empty pipe.
If the pipes are the same size and shape, the pipe full of sand has higher resistance to flow. Resistance, however, is not determined by the presence or absence of sand, it depends on the length and width of the pipe: short or wide pipes have lower resistance than narrow or long pipes. The above equation can be transposed to get Pouillet's law: R = ρ ℓ A; the resistance of a given material is proportional to the length, but inversely proportional to the cross-sectional area. Thus resistivity can be expressed using the SI unit "ohm metre" (i.e ohms divided by metres and multiplied by square metres }. For example, if A = 1 m2 ℓ = 1 m the resistance of this element in ohms is numerically equal to the resistivity of the material it is made of in Ω⋅m. Conductivity, σ, is the inverse of resistivity: σ = 1 ρ. Conductivity has SI units of "siemens per metre". For less ideal cases, such as more complicated geometry, or when the current and electric field vary in different parts of the material, it is necessary to use a more general expression in which the resistivity at a particular point is defined as the ratio of the electric field to the density of the current it creates at that point: ρ = E J, where ρ is the resistivity of the conductor material, E is the magnitude of the electric field, J is the magnitude of the current density,in which E and J are inside the conductor.
Conductivity is the inverse of resistivity. Here, it is given by: σ = 1 ρ = J E. For example, rubber is a material with large ρ and small σ—because a large electric field in rubber makes no current flow through it. On the other hand, copper is a material with small ρ and large σ—because a small electric field pulls a lot of current through it; as shown below, this expression simplifies to a single number when the electric field and current density are constant in the material. When the resistivity of a material has a directional component, the most general definition of resistivity must be used, it starts from the tensor-vector form of Ohm's law which relates the electric field inside a material to the electric current flow. This equation is general, meaning it is valid in all cases, including those mentioned above. However, this definition is the most complicated, so it is only directly used in anisotropic cases, where the more simple definitions cannot be applied. If the material is not anisotropic
Helium is a chemical element with symbol He and atomic number 2. It is a colorless, tasteless, non-toxic, monatomic gas, the first in the noble gas group in the periodic table, its boiling point is the lowest among all the elements. After hydrogen, helium is the second lightest and second most abundant element in the observable universe, being present at about 24% of the total elemental mass, more than 12 times the mass of all the heavier elements combined, its abundance is similar in Jupiter. This is due to the high nuclear binding energy of helium-4 with respect to the next three elements after helium; this helium-4 binding energy accounts for why it is a product of both nuclear fusion and radioactive decay. Most helium in the universe is helium-4, the vast majority of, formed during the Big Bang. Large amounts of new helium are being created by nuclear fusion of hydrogen in stars. Helium is named for the Greek Titan of the Sun, Helios, it was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by Georges Rayet, Captain C. T. Haig, Norman R. Pogson, Lieutenant John Herschel, was subsequently confirmed by French astronomer Jules Janssen.
Janssen is jointly credited with detecting the element along with Norman Lockyer. Janssen recorded the helium spectral line during the solar eclipse of 1868 while Lockyer observed it from Britain. Lockyer was the first to propose; the formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite. In 1903, large reserves of helium were found in natural gas fields in parts of the United States, by far the largest supplier of the gas today. Liquid helium is used in cryogenics in the cooling of superconducting magnets, with the main commercial application being in MRI scanners. Helium's other industrial uses—as a pressurizing and purge gas, as a protective atmosphere for arc welding and in processes such as growing crystals to make silicon wafers—account for half of the gas produced. A well-known but minor use is as a lifting gas in airships; as with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice.
In scientific research, the behavior of the two fluid phases of helium-4 is important to researchers studying quantum mechanics and to those looking at the phenomena, such as superconductivity, produced in matter near absolute zero. On Earth it is rare—5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements, as the alpha particles emitted by such decays consist of helium-4 nuclei; this radiogenic helium is trapped with natural gas in concentrations as great as 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation. Terrestrial helium—a non-renewable resource, because once released into the atmosphere it escapes into space—was thought to be in short supply. However, recent studies suggest that helium produced deep in the earth by radioactive decay can collect in natural gas reserves in larger than expected quantities, in some cases having been released by volcanic activity.
The first evidence of helium was observed on August 18, 1868, as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India; this line was assumed to be sodium. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D3 because it was near the known D1 and D2 Fraunhofer line lines of sodium, he concluded. Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος. In 1881, Italian physicist Luigi Palmieri detected helium on Earth for the first time through its D3 spectral line, when he analyzed a material, sublimated during a recent eruption of Mount Vesuvius. On March 26, 1895, Scottish chemist Sir William Ramsay isolated helium on Earth by treating the mineral cleveite with mineral acids. Ramsay was looking for argon but, after separating nitrogen and oxygen from the gas liberated by sulfuric acid, he noticed a bright yellow line that matched the D3 line observed in the spectrum of the Sun.
These samples were identified as helium by Lockyer and British physicist William Crookes. It was independently isolated from cleveite in the same year by chemists Per Teodor Cleve and Abraham Langlet in Uppsala, who collected enough of the gas to determine its atomic weight. Helium was isolated by the American geochemist William Francis Hillebrand prior to Ramsay's discovery when he noticed unusual spectral lines while testing a sample of the mineral uraninite. Hillebrand, attributed the lines to nitrogen, his letter of congratulations to Ramsay offers an interesting case of discovery and near-discovery in science. In 1907, Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei by allowing the particles to penetrate the thin glass wall of
A tokamak is a device which uses a powerful magnetic field to confine a hot plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power; as of 2016, it is the leading candidate for a practical fusion reactor. Tokamaks were conceptualized in the 1950s by Soviet physicists Igor Tamm and Andrei Sakharov, inspired by a letter by Oleg Lavrentiev. Meanwhile, the first working tokamak was attributed to the work of Natan Yavlinskii on the T-1, it had been demonstrated that a stable plasma equilibrium requires magnetic field lines that wind around the torus in a helix. Devices like the z-pinch and stellarator demonstrated serious instabilities, it was the development of the concept now known as the safety factor that guided tokamak development. The first tokamak, the T-1, began operation in 1958. By the mid-1960s, the tokamak designs began to show improved performance. Initial results were ignored. A second set of results was published in 1968, this time claiming performance far in advance of any other machine, was considered unreliable.
This led to the invitation of a delegation from the United Kingdom to make their own measurements. These confirmed the Soviet results, their 1969 publication resulted in a stampede of tokamak construction. By the mid-1970s, dozens of tokamaks were in use around the world. By the late 1970s, these machines had reached all of the conditions needed for practical fusion, although not at the same time nor in a single reactor. With the goal of breakeven now in sight, a new series of machines were designed that would run on a fusion fuel of deuterium and tritium; these machines, notably the Joint European Torus, Tokamak Fusion Test Reactor and JT-60, had the explicit goal of reaching breakeven. Instead, these machines demonstrated new problems. Solving these would require a much larger and more expensive machine, beyond the abilities of any one country. After an initial agreement between Ronald Reagan and Mikhail Gorbachev in November 1985, the International Thermonuclear Experimental Reactor effort emerged and remains the primary international effort to develop practical fusion power.
Many smaller designs, offshoots like the spherical tokamak, continue to be used to investigate performance parameters and other issues. The word tokamak is a transliteration of the Russian word токамак, an acronym of either: "тороидальная камера с магнитными катушками" — toroidal chamber with magnetic coils; the term was created in 1957 by Igor Golovin, the vice-director of the Laboratory of Measuring Apparatus of Academy of Science, today's Kurchatov Institute. A similar term, "tokamag", was proposed for a time. In 1934, Mark Oliphant, Paul Harteck and Ernest Rutherford were the first to achieve fusion on Earth, using a particle accelerator to shoot deuterium nuclei into a metal foil containing deuterium or other atoms; this allowed them to measure the nuclear cross section of various fusion reactions, determined that the deuterium-deuterium reaction occurred at a lower energy than other reactions, peaking at about 100,000 electronvolts. Accelerator-based fusion is not practical. To maintain fusion, the bulk of the fuel must be raised to high temperatures so its atoms are colliding at high speed.
In 1944, Enrico Fermi calculated the reaction would be self-sustaining at about 50,000,000 K. During the Manhattan Project, the first practical way to reach these temperatures was created, using an atomic bomb. In 1944, Fermi gave a talk on the physics of fusion in the context of a then-hypothetical hydrogen bomb. However, some thought had been given to a controlled fusion device, Jim Tuck and Stanislaw Ulam had attempted such using shaped charges driving a metal foil infused with deuterium, although without success; the first attempts to build a practical fusion machine took place in the United Kingdom, where George Paget Thomson had selected the pinch effect as a promising technique in 1945. After several failed attempts to gain funding, he gave up and asked two graduate students, Stan Cousins and Alan Ware, to build a device out of surplus radar equipment; this was operated in 1948, but showed no clear evidence of fusion and failed to gain the interest of the Atomic Energy Research Establishment.
In 1950, Oleg Lavrentiev a Red Army sergeant stationed on Sakhalin with little to do, wrote a letter to the Central Committee of the Communist Party of the Soviet Union. The letter outlined the idea of using an atomic bomb to ignite a fusion fuel, went on to describe a system that used electrostatic fields to contain a hot plasma in a steady state for energy production; the letter was sent to Andrei Sakharov for comment, who noted
Nuclear reactor core
A nuclear reactor core is the portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place and the heat is generated. The fuel will be low-enriched uranium contained in thousands of individual fuel pins; the core contains structural components, the means to both moderate the neutrons and control the reaction, the means to transfer the heat from the fuel to where it is required, outside the core. Inside the core of a typical pressurized water reactor or boiling water reactor are nuclear fuel rods equivalent to the diameter of a large gel type ink-pen, each about 4 m long, which are grouped by the hundreds in bundles called "fuel assemblies". Inside each fuel rod, pellets of uranium, or more uranium oxide, are stacked end to end. Inside the core are control rods, filled with pellets of substances like boron or hafnium or cadmium that capture neutrons; when the control rods are lowered into the core, they absorb neutrons, which thus cannot take part in the chain reaction.
Conversely, when the control rods are lifted out of the way, more neutrons strike the fissile uranium-235 or plutonium-239 nuclei in nearby fuel rods, the chain reaction intensifies. The core shroud located inside of the reactor, directs the water flow to cool the nuclear reactions inside of the core; the heat of the fission reaction is removed by the water, which acts to moderate the neutron reactions. An alternative form of nuclear fuel would be fissile uranium-233 made by the neutron-bombardment of the common thorium-232. There are Graphite moderated reactors in use. One type uses ordinary water for the coolant. See the Soviet-made RBMK nuclear-power reactor; this was the type of reactor involved in the Chernobyl disaster. In the advanced gas-cooled reactor, a British design, the core is made of a graphite neutron moderator where the fuel assemblies are located. Carbon dioxide gas acts as a coolant and it circulates through the core, removing heat. There have been several experimental reactors that use graphite for moderation, such as the pebble bed reactor concepts and the molten-salt reactor experiment.
Nuclear meltdown Lists of nuclear disasters and radioactive incidents Nuclear power Nuclear reactor technology Nuclear Reactor Analysis, John Wiley & Sons Canada, Ltd
A magnetic field is a vector field that describes the magnetic influence of electric charges in relative motion and magnetized materials. Magnetic fields are observed from subatomic particles to galaxies. In everyday life, the effects of magnetic fields are seen in permanent magnets, which pull on magnetic materials and attract or repel other magnets. Magnetic fields surround and are created by magnetized material and by moving electric charges such as those used in electromagnets. Magnetic fields exert forces on nearby moving electrical torques on nearby magnets. In addition, a magnetic field that varies with location exerts a force on magnetic materials. Both the strength and direction of a magnetic field vary with location; as such, it is an example of a vector field. The term'magnetic field' is used for two distinct but related fields denoted by the symbols B and H. In the International System of Units, H, magnetic field strength, is measured in the SI base units of ampere per meter. B, magnetic flux density, is measured in tesla, equivalent to newton per meter per ampere.
H and B differ in. In a vacuum, B and H are the same aside from units. Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. Magnetic fields and electric fields are interrelated, are both components of the electromagnetic force, one of the four fundamental forces of nature. Magnetic fields are used throughout modern technology in electrical engineering and electromechanics. Rotating magnetic fields are used in both electric generators; the interaction of magnetic fields in electric devices such as transformers is studied in the discipline of magnetic circuits. Magnetic forces give information about the charge carriers in a material through the Hall effect; the Earth produces its own magnetic field, which shields the Earth's ozone layer from the solar wind and is important in navigation using a compass. Although magnets and magnetism were studied much earlier, the research of magnetic fields began in 1269 when French scholar Petrus Peregrinus de Maricourt mapped out the magnetic field on the surface of a spherical magnet using iron needles.
Noting that the resulting field lines crossed at two points he named those points'poles' in analogy to Earth's poles. He clearly articulated the principle that magnets always have both a north and south pole, no matter how finely one slices them. Three centuries William Gilbert of Colchester replicated Petrus Peregrinus' work and was the first to state explicitly that Earth is a magnet. Published in 1600, Gilbert's work, De Magnete, helped to establish magnetism as a science. In 1750, John Michell stated that magnetic poles attract and repel in accordance with an inverse square law. Charles-Augustin de Coulomb experimentally verified this in 1785 and stated explicitly that the north and south poles cannot be separated. Building on this force between poles, Siméon Denis Poisson created the first successful model of the magnetic field, which he presented in 1824. In this model, a magnetic H-field is produced by'magnetic poles' and magnetism is due to small pairs of north/south magnetic poles. Three discoveries in 1820 challenged this foundation of magnetism, though.
Hans Christian Ørsted demonstrated that a current-carrying wire is surrounded by a circular magnetic field. André-Marie Ampère showed that parallel wires with currents attract one another if the currents are in the same direction and repel if they are in opposite directions. Jean-Baptiste Biot and Félix Savart announced empirical results about the forces that a current-carrying long, straight wire exerted on a small magnet, determining that the forces were inversely proportional to the perpendicular distance from the wire to the magnet. Laplace deduced, but did not publish, a law of force based on the differential action of a differential section of the wire, which became known as the Biot–Savart law. Extending these experiments, Ampère published his own successful model of magnetism in 1825. In it, he showed the equivalence of electrical currents to magnets and proposed that magnetism is due to perpetually flowing loops of current instead of the dipoles of magnetic charge in Poisson's model.
This has the additional benefit of explaining. Further, Ampère derived both Ampère's force law describing the force between two currents and Ampère's law, like the Biot–Savart law described the magnetic field generated by a steady current. In this work, Ampère introduced the term electrodynamics to describe the relationship between electricity and magnetism. In 1831, Michael Faraday discovered electromagnetic induction when he found that a changing magnetic field generates an encircling electric field, he described this phenomenon in. Franz Ernst Neumann proved that, for a moving conductor in a magnetic field, induction is a consequence of Ampère's force law. In the process, he introduced the magnetic vector potential, shown to be equivalent to the underlying mechanism proposed by Faraday. In 1850, Lord Kelvin known as William Thomson, distinguished between two magnetic fields now denoted H and B; the former applied to the latter to Ampère's model and induction. Further, he derived how H and B relate to each other
Uranium hexafluoride, colloquially known as "hex" in the nuclear industry, is a compound used in the process of enriching uranium, which produces fuel for nuclear reactors and nuclear weapons. Hex forms solid grey crystals at standard temperature and pressure, is toxic, reacts with water, is corrosive to most metals; the compound reacts mildly with aluminium, forming a thin surface layer of AlF3 that resists any further reaction from the compound. Milled uranium ore—U3O8 or "yellowcake"—is dissolved in nitric acid, yielding a solution of uranyl nitrate UO22. Pure uranyl nitrate is obtained by solvent extraction treated with ammonia to produce ammonium diuranate. Reduction with hydrogen gives UO2, converted with hydrofluoric acid to uranium tetrafluoride, UF4. Oxidation with fluorine yields UF6. During nuclear reprocessing, uranium is reacted with chlorine trifluoride to give UF6: U + 2 ClF3 → UF6 + Cl2 At atmospheric pressure, it sublimes at 56.5 °C. The solid state structure was determined by neutron diffraction at 77 K and 293 K.
It has been shown that uranium hexafluoride is an oxidant and a Lewis acid, able to bind to fluoride. Polymeric uranium fluorides containing organic cations have been isolated and characterised by X-ray diffraction. UF6 is used in both of the main uranium enrichment methods — gaseous diffusion and the gas centrifuge method — because its triple point is at temperature 64.05 °C and only higher than normal atmospheric pressure. Fluorine has only a single occurring stable isotope, so isotopologues of UF6 differ in their molecular weight based on the uranium isotope present. All the other uranium fluorides are nonvolatile solids. Gaseous diffusion requires about 60 times as much energy as the gas centrifuge process: gaseous diffusion-produced nuclear fuel produces 25 times more energy than is used in the diffusion process, while centrifuge-produced fuel produces 1,500 times more energy than is used in the centrifuge process. In addition to its use in enrichment, uranium hexafluoride has been used in an advanced reprocessing method, developed in the Czech Republic.
In this process, used oxide nuclear fuel is treated with fluorine gas to form a mixture of fluorides. This mixture is distilled to separate the different classes of material. Uranium enrichment produces large quantities of depleted uranium hexafluoride, or DUF6, as a waste product; the long-term storage of DUF6 presents environmental and safety risks because of its chemical instability. When UF6 is exposed to moist air, it reacts with the water in the air to produce UO2F2 and HF both of which are corrosive and toxic. In 2005, 686,500 tonnes of DUF6 was housed in 57,122 storage cylinders located near Portsmouth, Ohio. Storage cylinders must be inspected for signs of corrosion and leaks; the estimated lifetime of the steel cylinders is measured in decades. There have been several accidents involving uranium hexafluoride in the US, including a cylinder-filling accident and material release at the Sequoyah Fuels Corporation in 1986; the U. S. government has been converting DUF6 to solid uranium oxides for disposal.
Such disposal of the entire DUF6 inventory could cost anywhere from $15 million to $450 million. Gmelins Handbuch der anorganischen Chemie, System Nr. 55, Teil A, p. 121–123. Gmelins Handbuch der anorganischen Chemie, System Nr. 55, Teil C 8, p. 71–163. R. DeWitt: Uranium hexafluoride: A survey of the physico-chemical properties, Technical report, GAT-280. Portsmouth, Ohio. August 1960. Ingmar Grenthe, Janusz Drożdżynński, Takeo Fujino, Edgar C. Buck, Thomas E. Albrecht-Schmitt, Stephen F. Wolf: Uranium, in: Lester R. Morss, Norman M. Edelstein, Jean Fuger: The Chemistry of the Actinide and Transactinide Elements, Dordrecht 2006. US-Patent 2535572: Preparation of UF6. December 1950. US-Patent 5723837: Uranium Hexafluoride Purification. March 1998. Simon Cotton: Uranium Hexafluoride. Uranium Hexafluoride – Physical and chemical properties of UF6, its use in uranium processing – Uranium Hexafluoride and Its Properties Import of Western depleted uranium hexafluoride to Russia Uranium Hexafluoride in www.webelements.com
A nuclear reactor known as an atomic pile, is a device used to initiate and control a self-sustained nuclear chain reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships. Heat from nuclear fission is passed to a working fluid; these either turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating; some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. Some are run only for research; as of early 2019, the IAEA reports there are 454 nuclear power reactors and 226 nuclear research reactors in operation around the world. Just as conventional power-stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear reactors convert the energy released by controlled nuclear fission into thermal energy for further conversion to mechanical or electrical forms; when a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission.
The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation, free neutrons. A portion of these neutrons may be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, so on; this is known as a nuclear chain reaction. To control such a nuclear chain reaction, neutron poisons and neutron moderators can change the portion of neutrons that will go on to cause more fission. Nuclear reactors have automatic and manual systems to shut the fission reaction down if monitoring detects unsafe conditions. Used moderators include regular water, solid graphite and heavy water; some experimental types of reactor have used beryllium, hydrocarbons have been suggested as another possibility. The reactor core generates heat in a number of ways: The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms; the reactor absorbs some of the gamma rays produced during fission and converts their energy into heat.
Heat is produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat-source will remain for some time after the reactor is shut down. A kilogram of uranium-235 converted via nuclear processes releases three million times more energy than a kilogram of coal burned conventionally. A nuclear reactor coolant — water but sometimes a gas or a liquid metal or molten salt — is circulated past the reactor core to absorb the heat that it generates; the heat is carried away from the reactor and is used to generate steam. Most reactor systems employ a cooling system, physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor. However, in some reactors the water for the steam turbines is boiled directly by the reactor core; the rate of fission reactions within a reactor core can be adjusted by controlling the quantity of neutrons that are able to induce further fission events.
Nuclear reactors employ several methods of neutron control to adjust the reactor's power output. Some of these methods arising from the physics of radioactive decay and are accounted for during the reactor's operation, while others are mechanisms engineered into the reactor design for a distinct purpose; the fastest method for adjusting levels of fission-inducing neutrons in a reactor is via movement of the control rods. Control rods therefore tend to absorb neutrons; when a control rod is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces—often the moderator. This action results in fewer neutrons available to cause fission and reduces the reactor's power output. Conversely, extracting the control rod will result in an increase in the rate of fission events and an increase in power; the physics of radioactive decay affects neutron populations in a reactor. One such process is delayed neutron emission by a number of neutron-rich fission isotopes; these delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder released upon fission.
The fission products which produce delayed neutrons have half lives for their decay by neutron emission that range from milliseconds to as long as several minutes, so considerable time is required to determine when a reactor reaches the critical point. Keeping the reactor in the zone of chain-reactivity where delayed neutrons are necessary to achieve a critical mass state allows mechanical devices or human operators to control a chain reaction in "real time"; this last stage, where delayed neutrons are no longer required to maintain criticality, is known as the prompt critical point. There is a scale for describing criticality in numerical form, in which bare criticality is known as zero dollars and the prompt critical point is one dollar, other points in the process interpolated in cents. In some reactors, the coolant acts as a neutron moderator. A moderator increases the power of the reactor by causin